An analysis reports that the stepping force applied to a brake pedal by a driver during the emergency brake is smaller than the stepping force necessary for generating the sufficient brake hydraulic pressure for locking wheels and thus the sufficient braking performance of the vehicle is not preformed during the emergency brake that the driver suddenly steps on the brake pedal for brake depression when obstacles are suddenly appeared on the way.
In late years, devices for increasing the brake hydraulic pressure to the degree for locking the wheels during the emergency brake based on the stepping speed of the brake pedal for judging whether it is a normal brake for performing the braking operating under the normal condition or it is an emergency brake for operating the brake operation under the emergent condition have been proposed to be implemented.
A known device (i.e., vacuum booster) for assisting the brake operation for the emergency, WO 01/32488 A1 is disclosed. The known device for assisting the brake operation includes a housing formed with at least one pressure chamber therein, a movable wall which is provided in the housing for moving forward and moving backward relative to the housing and which divides the pressure chamber into a constant pressure chamber and a variable pressure chamber, a power piston connected to the movable wall, an input rod provided in the power piston for moving forward and moving backward relative to the power piston, and a plunger connected to the input rod to be inserted into the power piston for being slidable in the longitudinal direction relative to the power piston and formed with an atmosphere valve seat on the rear end portion thereof. The known device for assisting the brake operation further includes an output member for outputting the driving force of the power piston accompanied with the movement of the movable wall, a first vacuum valve seat provided on the power piston facing rearward, a second vacuum valve seat which can move forward and backward relative to the power piston and formed on the valve seat member which is approximately co-axial to the first vacuum valve seat, a valve mechanism having a movable portion facing the first vacuum valve seat and a fixing portion air-tightly fixed to the power piston, a drive means for moving a valve seat member towards the movable portion by a predetermined amount when the forward moving amount of the plunger relative to the power piston is equal to or greater than a predetermined value, a holding means for holding the valve seat member at a predetermined position in the power piston when the forward moving amount of the plunger relative to the power piston is smaller than the predetermined value, an engaging member fixed to the power piston for engaging the holding means to the power piston, and a key member inserted into a radial directional bore of the power piston for restricting the moving amount in longitudinal direction of the plunger relative to the power piston. The plunger includes a first flange portion fitted into a penetration bore provided on the valve seat member and a second flange portion formed on the rear of the first flange portion and fitted into an internal peripheral surface of the power piston. The key member is provided between the first flange portion and the second flange portion.
With the known device for assisting the brake operation with the foregoing construction, when the forward moving speed of the plunger is greater than the forward moving speed of the power piston due to the sudden depression of the brake pedal by the driver's operation, that is, when the relative moving amount in the forward direction of the plunger relative to the power piston becomes equal to or greater than the predetermined value, the engagement between the holding member and the valve seat member is released to move the valve seat member backward. By extending a distance between a rear end surface of a reaction member (i.e., reaction disc) and a front-end surface of a valve sealing portion of the valve mechanism, as it called jumping amount is increased to obtain the emergency brake characteristics.
That is, under the emergency brake operation, by switching an input-output characteristic of the vacuum booster device from a normal brake characteristics to an emergency brake characteristic, the braking characteristics the same with under the condition that the driver strongly steps on the brake pedal to be strongly depressed can be obtained.
With the known vacuum booster disclosed in the WO 01/32488 A1,the valve seat member is moved in the backward direction (i.e., rearward) when the relative moving amount of the plunger in the forward direction relative to the power piston becomes equal to or greater than the predetermined value under the emergency brake operation and when the engagement between the holding member and the valve seat member is released. Accompanying with the backward movement of the valve seat member, the key member fitted into a slit of the valve seat member is moved backward being pushed by a front groove wall of the slit of the valve seat member and the key member pushed backward contacts the front end surface of the plunger. In this case, for example, in case the key member and the plunger are made of hard material such as metal, the operational noise is generated between the key member and the plunger to decline the quality of the vacuum booster.
A need thus exists for a vacuum booster which reduces the operational noise under the emergency brake operation.